1. Field of the Invention
The present invention relates to a loop antenna suitable for a mobile communication terminal such as a cellular phone or PDA (Personal Digital Assistants) and a radio communication device having the loop antenna.
2. Description of the Related Art
Recent mobile communication terminals such as cellular phones and PDAs use internal antennas having an antenna element accommodated in a housing from the viewpoint of further size reduction and design of terminals. However, an internal antenna readily degrades its performance in the voice communication posture, as compared to an antenna arranged outside the housing.
The antenna performance can deteriorate in the voice communication posture due to two reasons below.
(1) During voice communication, the terminal housing readily comes close to the speaker as a lossy dielectric medium. Since the transmission radio wave absorption amount in the lossy dielectric medium increases, the radiation efficiency decreases.
(2) During voice communication, the terminal housing is often tilted obliquely or horizontally. For this reason, the reception efficiency for a vertical polarized wave arriving from the base station decreases.
In addition, the antenna radiation pattern changes depending on the tilt angle of the terminal housing in the voice communication posture. This also poses a problem in maintaining a stable voice communication state.
So use of a traditional small circular loop antenna could be preliminary approach to examine. A small circular loop antenna is formed by a 0.1-wavelength antenna segment having a ring shape. With this antenna, a radiation pattern with radiation suppressed in a direction toward the speaker can be obtained. In addition, a predetermined antenna gain can be held independently of the tilt angle of the terminal housing during voice communication. However, since the small circular loop antenna has a short circumferential length, the radiation resistance is low, and the aperture area is small. For this reason, impedance matching to a radio circuit is difficult to ensure.
On the other hand, as an internal antenna of another type suitable for mobile communication terminals, a dipole antenna which has a Z- or H-shaped segment and supplies power at the central segment portion has been proposed in, e.g., U.S. Pat. No. 5,767,809 or Chi-Chang, et al, “A 2.4 GHz Omni-directional Horizontally Polarized Planar Printed Antenna for WLAN Applications”, 2003 IEEE. An antenna of this type can obtain a radiation pattern similar to that of a small circular loop antenna. In addition, impedance matching to a radio circuit can easily be ensured.
However, an antenna of this type has a segment at the antenna central portion and supplies power on the central segment. It is therefore difficult to use this antenna in a radio communication device having a large circuit component mounted at the central portion of the housing, like a folding cellular phone having a back display.
As described above, the conventionally developed or proposed internal antennas can hardly obtain impedance matching to a radio circuit because of their low radiation resistance and small aperture area. In addition, since a central segment and power supply at the central portion of the segment are necessary, the degree of freedom in mounting is low. For this reason, the antennas are not appropriate for compact radio communication devices having many restrictions on mounting, like a cellular phone having a back display.